Primers require a sharp blow to detonate, too. You'd have to stomp on one, not just "step" on it.
If you set it up right, with a rock under the nail and dirt packed tightly around the shell AND the BG stomped on it, like he was running after you, I'd be willing to bet that it would be bad news for his foot. No where near full-power but I don't think it would be good.
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.